Question

If using the method of completing the square to solve the quadratic equation

x^2– 10x + 21 = 0, which number would have to be added to "complete the
square"?

Answer 1

Answer: 25

Step-by-step explanation: The number that is added to both sides is the number that is needed to create a perfect square trinomial.

The question is, what is that number?

Well, it comes from a formula. The number that is added to both sides always comes from half the coefficient of the middle term squared.

So half of -10 squared or -5 squared which is 25.

So 25 is added to both sides.